us is an alternative interface to the Sia network. It provides low-level, developer-oriented APIs and formats that facilitate the storage and retrieval of files on Sia. "Low-level" means that us generally avoids making decisions on behalf of the user. For example, when renting storage, the user must decide which hosts to form contracts with, how many coins to spend on each contract, when to renew contracts, when to migrate data to new hosts, etc. These questions do not have simple answers; they vary according to the context and goals of the user. Recognizing this, the us philosophy is to provide the user with a set of building blocks rather than a one-size-fits-all solution. us enables you to do a number of cool things with Sia that were previously infeasible, difficult, or inefficient. You can specify exactly which hosts you want to use. You can collect data on exactly how much you paid to download a file, and exactly how long it took the host to transfer it to you. You can use this information to manually blacklist hosts, selecting only the cheapest or fastest hosts when downloading. You can "pack" multiple files into a single uploaded sector: if you're uploading an album of JPEGs averaging 250 KB each, this reduces storage and bandwidth costs by about 16x. You can download one of those JPEGs without downloading the full sector, too. You can trivially share files, just by sending the metadata to a friend. You can stream files over HTTP or mount a virtual Sia filesystem with FUSE, enjoying low latency as a result of partial downloads. You can upload and download without needing to run a full consensus node. And most importantly, you can build apps that leverage Sia to store and serve content.